A solar cell module has a plurality of silicon cells interconnected via lead wires. The lead wires are also called as interconnectors, and are formed by solder-plating a flat rectangular wire.
The interconnectors are connected to the silicon cells via the solder plate. However, the interconnectors and the silicon cells differ in thermal expansion coefficient. Therefore, bending stress may be generated in the silicon cell that has a smaller thermal expansion coefficient, due to the influence of heat at the time of soldering. Because of the differences in rigidity and surface area between the interconnector and the solar cell, the interconnector deforms earlier than the solar cell by the heat treatment at the time of soldering. The strain of the interconnector causes warping or breakage of the silicon cell. To address this problem, there is a need for reducing 0.2% proof stress of the interconnector. The 0.2% proof stress is an index of mechanical properties. The smaller the 0.2% proof stress of the interconnector, the more the warping of the silicon cell can be reduced.
Interconnectors are manufactured by flattening a conductor material using a die or a roller, slitting the flattened conductor material to form a thin wire having a rectangular sectional shape, heating the conductor wire, and solder plating the conductor wire.
The heat treatment after the slitting process is an annealing process, and removes the internal strain of the conductor wire that has undergone the flattening process and the slitting process, and softens the structure.
To reduce the 0.2% proof stress of a conductor wire having a flat rectangular sectional shape, an indirect heating is being proposed as a heat treatment method (see, e.g., JP2009-016593A, JP2009-027096A, JP2009-280898A and JP2010-141050A).
As for the solder-plating, a bath containing solder in a heated and molten state, a die disposed near the surface level of the bath, and a turn roll disposed inside the bath to rotate about a horizontal axis may be used (see. e.g., JP2010-095750A and JP2011-058051A).